Officially, Season 2 is a sequel to Season 1; it was released after Season 1 and doesn't repeat any material. The US DVD releases for the two are completely separate. But because of the jumbled continuity of the Haruhi Suzumiya series, the Japanese TV broadcast of Season 2 included a rebroadcast of Season 1. That's why when you looked it up, your source said there were 28 episodes; it was going by the TV broadcast, and wasn't being careful about the distinction between Season 1 and Season 2.
Since the timelines of Seasons 1 and 2 are interleaved, when Season 2 was broadcast on Japanese TV, it was shown along with a rebroadcast of Season 1, with all the episodes in chronological order. For example, The Sigh of Haruhi Suzumiya (in Season 2) was played before "Live Alive" (in Season 1) because The Sigh takes place before "Live Alive". (The movie the Brigade shows at the culture festival during "Live Alive" was filmed during The Sigh.)
The merging of Season 1 and Season 2 was only for the TV broadcast; if you buy the DVDs, they're separate, so if you haven't bought the Season 1 DVDs yet, make sure to get those along with the Season 2 set. The Season 2 DVD set only includes the new episodes. The DVD sets don't have the episodes chronologically ordered; they have their own ordering for the episodes which is different from both the chronological ordering and the TV broadcast. You can read more about that on the Wikipedia page, but I'll give a little summary:
- Season 1 was originally broadcast in weirdo order, then later broadcast in chronological order without the Season 2 episodes.
- When Season 2 came out, it was shown along with the episodes from Season 1; the entire series was shown in chronological order for this broadcast.
- The US DVD sets have their own ordering. The Season 1 DVD sets are ordered chronologically amongst themselves, except that "The Adventures of Mikuru Asahina Episode 00" is the first episode. The Season 2 DVD sets are ordered chronologically amongst themselves.
(I'm not even going to go into whether the various repetitions of "Endless Eight" have a chronological ordering or not--the metaphysics of time is off topic here.)